The major joints of the human body are subject to high mechanical stresses. For example, the joints of the locomotor apparatus have to bear a large part of the body's weight, and moreover they are moved every time a step is taken. Therefore, the bones which support the joints have a powerful cortical structure. Their integrity is important for sufficient functioning of the joint. The same is true of the arm joints; although the weight which they have to support is lower, they are moved more frequently and are therefore exposed to high levels of wear. Moreover, their dimensions are smaller and they are more susceptible to injury.
Prostheses intended for permanent implantation (endoprostheses) not only have to have sufficient mechanical properties to ensure the desired functionality, but also have to have a biocompatibility that is as high as possible to ensure that they are tolerated by the patient over a prolonged period of time. In particular the latter aspect is very important, since any incompatibilities which occur generally require explantation of the prosthesis. This equates to failure of the prosthesis.
It is known that inadequate transmission of load from the prosthesis to the surrounding bone can lead to degeneration of the bone tissue. This often leads to the prosthesis coming loose. Therefore, to avoid this degeneration, it is important to ensure loading that is as physiological as possible by the prosthesis. Tests have shown that hip prostheses with a lower modulus of elasticity produce a loading situation which is more physiological than when using rigid prostheses. For example, in the case of femoral prostheses, there has been a move away from cobalt-chromium alloys, which generally have a very high modulus of elasticity in the region of approx. 200 000 N/mm2, toward titanium alloys, which have a lower modulus of elasticity, such as for example TiAl6V4, the modulus of which is approx. 100 000 N/mm2. However, these levels are still well above the modulus of elasticity of the cortical bone, at approx. 25 000 N/mm2.